Time-Constrained Web Services for Industrial Automation
Service-oriented architectures based on web services have become the de facto standard for the design and implementation of distributed applications. Due to their standardization, web services are widely adopted for inter-organizational communication, but their use for intra-organizational communica...
Mathematik und Informatik
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|Summary:||Service-oriented architectures based on web services have become the de facto standard for the design and implementation of distributed applications. Due to their standardization, web services are widely adopted for inter-organizational communication, but their use for intra-organizational communication is challenging, especially within enterprises of the manufacturing domain. Business processes within an industrial enterprise have to satisfy predefined time constraints. More precisely, the manufacturing process has to guarantee real-time, i.e. predefined deadlines have to be kept. Todays industrial enterprises use specialized hard- and software at the manufacturing layer - industrial PCs (IPCs) and programmable logic controllers (PLCs) - that run under vendor-specific operating systems and are implemented using vendor-specific tools. This mixture of technologies of different vendors leads to numerous breaks in the communication paradigm. The interconnection of these technologies results in additional costs and is often quite error-prone.
This thesis investigates the seamless use of web services as a homogeneous communication backbone throughout the overall industrial enterprise with a focus on the manufacturing layer. The common layered organization of an industrial enterprise - consisting of the business layer, the intermediate layer, and the manufacturing layer - is avoided by using web services as the communication backbone. The need for real-time makes the use of web services in industrial automation challenging. Web service standards (e.g. SOAP for the invocation and WSDL for the description of web services) and technologies (e.g. available SOAP engines) have emerged from the Internet domain where processing is based on a best-effort basis (time constraints are of little or no interest).
The Time-Constrained Services (TiCS) framework presented in this thesis is a technical foundation for using web services in time-constrained environments with a specific focus on industrial automation. TiCS is a tool suite for the development, deployment, publication, composition, and execution of web services with time constraints, especially real-time constraints. TiCS consists of several components to ease the entire development process for automation engineers who are normally not familiar with web service technologies. The main components of the TiCS framework can be distinguished according to their functionality:
- permit access to the manufacturing layer using web services
- permit the composition of several web services to a value-added workflow that represents a production process
- offer a mechanism to describe the time constraints of web services and workflows
- permit the efficient transmission of binary web service parameters
To access the manufacturing layer using web services, the TiCS framework offers a real-time SOAP engine for IPCs called SOAP4IPC and for PLCs called SOAP4PLC. Both engines permit an evolutionary change to a web service based communication backbone, since existing hardware can be used further.
Most real-world production processes consist of several steps. Due to this reason, it is necessary to combine web services to a multi-step workflow. The TiCS Modeler supports an automation engineer to define such workflows with regard to the required time constraints.
The WS-TemporalPolicy language permits the description of time constraints for web services and entire workflows. Since time constraints may vary over time, e.g. compare processing during peak time and off-peak time, WS-TemporalPolicy permits the definition of a validity period for time constraints.
The protocol predominantly used for the invocation of web services is SOAP. Since SOAP is based on XML, it is necessary to encode binary parameters, resulting in a remarkable overhead. For an efficient parameter transmission - a fundamental prerequisite to process a web service within a given deadline - the TiCS framework contains a component called Flex-SwA.
Despite the fact that the technologies developed in this thesis focus on industrial automation, they can also be used in related research areas where web services are used as the communication technology.|